In recent years, various porous honeycomb filters have been proposed as porous ceramic filters, wherein partitions of the honeycomb structure made of cordierite are allowed to have a porous structure, and the porous honeycomb filters are thereby made to have a function of filtrating fluids, e.g. gases, by passing the fluids through the partitions. Examples thereof in practical use include a filter for collecting fine particles in an exhaust gas discharged from a diesel car (diesel particulate filter).
In such a porous honeycomb filter, an average pore diameter (hereafter referred to as pore diameter) and a porosity of the porous material are very important factors for determining the performance of the filter. In particular, with respect to a porous ceramic filter, such as a diesel particulate filter, it is desired that the filter has a large pore diameter and a large porosity from the viewpoint of collection efficiency of fine particles, pressure drop and collection time.
The pore diameter is conventionally controlled by appropriately selecting a particle diameter of an aggregate in a ceramic composition to become a material for a filter. However, there are problems with the material cost and the productivity due to restrictions on aggregate particles.
As a method for controlling the pore diameter under no influence of the type of aggregate particle, for example, a method in which an organic macromolecule is added to a ceramic composition is proposed (Japanese Unexamined Patent Application Publication No. 2000-288325).
On the other hand, a pore-forming agent, e.g. graphite, is added to a ceramic composition in a common method for improving the porosity.
However, when large amounts of pore-forming agent is used in order to further improve the porosity, problems occur in that a firing time is extended, the number of steps is increased and, in addition, a strain is put on the filter due to an increase in the heat of combustion of the pore-forming agent, and cracks thereby occur in the filter.
That is, in the ceramic filter, reduction in thermal expansion and an improvement in thermal-shock resistance are important.
In order to reduce the heat of combustion, the inventors of the present invention considered the use of hollow polymer particles as pore-forming agents. However, problems occurred in that conventional hollow polymer particles were too small for use as the pore-forming agents, and that since hollow polymer particles prepared by foaming had insufficient particle strength, particles were broken due to mechanical shearing force during mixing or molding of the ceramic composition. Therefore, no hollow polymer particle was available with a particle diameter and strength suited for performing a function as a pore-forming agent.
That is, with respect to the method for manufacturing hollow polymer particles, a method is known, in which suspension polymerization or emulsion polymerization is performed in the presence of a water-insoluble organic solvent having a non-polymerization property and, thereby, polymer particles including the solvent in the interior pores are prepared as disclosed in, for example, Japanese Examined Patent Application Publication No. 36-9168 and Japanese Examined Patent Application Publication No. 37-14327.
However, in such a method, there are problems in that particles including a desired solvent may not be sufficiently prepared, the distributions of particle diameters and diameters of hollow portions (interior pores) become significantly large and the like. Consequently, satisfactory hollow polymer particles cannot be prepared even when these included materials are removed.
On the other hand, as a means for improving this, a method is disclosed in Japanese Examined Patent Application Publication No. 5-40770. In the method, a mixed solution prepared by dispersing a hydrophilic monomer, a cross-linkable monomer and an oily material is subjected to suspension polymerization or emulsion polymerization so as to prepare polymer particles including the oily material in the interior pore and, thereafter, the oily material is removed, so that hollow polymer particles are prepared.
However, in this method, even when the amount of a polymerization dispersing agent is controlled in order to prepare hollow polymer particles having an average particle diameter of 15 μm or more, suitable for a pore-forming agent, coagulation of the particles is brought about, and therefore, it is difficult to prepare desired particles.
In the case disclosed in Japanese Unexamined Patent Application Publication No. 9-19635 where a microcapsule encapsulating a volatile material, e.g. butane or pentane, is heated, and the volatile material is thereby gasified to expand so as to prepare foamed particles, since the thickness of a shell polymer layer becomes very small, no particle having satisfactory strength can be prepared.
Some methods are known, in which polymer particles including hollow portions are manufactured without using the above-described organic solvents.
That is, for example, Japanese Unexamined Patent Application Publication No. 56-32513 discloses a method for preparing particles. In the method, a basic material is applied to a polymer particle composed of a core portion having a swelling property by an alkali and a shell portion covering the core portion so as to swell and expand the core portion, and a hollow portion is thereby formed in the particle during drying. Japanese Unexamined Patent Application Publication No. 2-173101 discloses a method in which a core portion of a core/shell type polymer having the core portion composed of a vinyl acetate polymer is hydrolyzed, and the method is similar to that disclosed in Japanese Unexamined Patent Application Publication No. 56-32513.
However, with respect to these methods, since the shell portion must also have plasticity in order to expand the core portion, there are problems in that, for example, polymer particles having excellent strength and hollowing portions cannot be prepared with ease, and since water absorbed in the polymer of the core portion is not removed with ease, the drying property is poor when used for a predetermined purpose.
Consequently, it is desired to provide hollow polymer particles which can be used as pore-forming agents for the above-described ceramic composition and which have a predetermined value or more of strength and a proper particle diameter.
It is an object of the present invention to overcome the above-described problems and to provide hollow polymer particles which can be effectively used as pore-forming agents and which have a predetermined value or more of strength and a proper particle diameter, a method for manufacturing the same, a high-performance porous ceramic filter manufactured using the hollow polymer particles and a method for manufacturing the same.